He is definitely from the former Yugoslavia.
I highly doubt that he's Croatian (they use the word Serb far to often in the intro for him to be a Croatian), but he could be either Serbian or Bosnian (a Bosnian Serb that is, doubt he's a Muslim or a Catholic).
And they speak Yugoslavian (or Serbian or Bosnian or Croatian, depending where you are from
) and not Russian or Polish.
It would also fit a stereotype well for many if the would use an "evil" Serbian.